Comparative study of optical phonons in the rhombohedrally distorted perovskites LaAlO3 and LaMnO3

The Raman and infrared phonons of isostructural rhombohedral LaMnO3 and LaAlO3 are studied at room temperature. The experimental spectra an compared with the prediction of lattice-dynamical calculations and the lines observed are assigned to definite atomic vibrations. It is shown that the Raman mode of A(1g) symmetry in LaAlO3 and LaMnO3 (at 123 cm(-1) and 236 cm(-1), respectively) involves atomic motions that cause the rhomboheciral distortion, i.e., it is a "soft'' mode, and its position could be used as a measure of the degree of the distortion. It is also argued that the broad Raman bands in the high-frequency range of LaMnO3 are not proper modes of the rhombohedral R (3) over bar e structure, but are rather induced by the dynamic Jahn-Teller effect. [S0163-1829(99)02706-X].